The invention relates to an improved focus mask type cathode-ray tube (CRT) and, more particularly, to an improved color-selection structure for such a tube having a first electrode comprising conductors forming a plurality of alternate columns of first and second apertures and a second electrode including an array of conductors which intersect the second apertures to form first and second windows. The invention further includes a method of producing a luminescent viewing screen using the improved color-selection structure.
A commercial shadow-mask-type CRT comprises generally an evacuated envelope having therein a target or viewing screen comprising an array of phosphor elements of three different emission colors arranged in a cyclic order, means for producing three convergent electron beams directed towards the target, and a color-selection structure including an apertured masking plate between the target and the beam-producing means. The masking plate shadows the target, and the differences in convergence angles permit the transmitted portions of each beam, or beamlets, to select and excite phosphor elements of the desired emission color.
At about the center of the color-selection structure, the masking plate of a commercial CRT intercepts all but about 18% of the beam currents; that is, the plate is said to have a transmission of about 18%. Thus, the area of the apertures of the plate is about 18% of the area of the plate. Since there are no focusing fields present, a corresponding portion of the target is excited by the beamlets of each electron beam.
Several methods have been suggested for increasing the transmission of the masking plate; that is, increasing the area of the apertures with respect to the area of the plate, without substantially increasing the excited portions of the target area. In one approach, the apertures are enlarged, and the beamlets are focused by magnetic or electric fields produced in the vicinity of each of the apertures. In a second approach, each aperture in the masking plate is enlarged and split into two adjacent windows by a conductor. The two beamlets passing through the windows of each aperture are deflected around the conductor towards one another, and both beamlets fall on substantially the same area of the target. In this second approach, the transmitted portions of the beams are also focused in one transverse direction and defocused in the orthogonal transverse direction.
One effort at such a combined deflection-and-focus color-selection means is described in West German Offenlegungschrift No. 2,814,391, published Oct. 19, 1978. The color-selection structure described therein comprises a metal masking plate having an array of substantially square apertures arranged in vertical columns and an array of narrow vertical conductors insulatingly spaced from the masking plate, with each conductor substantially centered over the apertures of one of the columns of apertures. Each aperture is also centered over a triad of phosphor stripes. Viewed from the electron-beam-producing means, the conductors divide each aperture into two essentially-equal horizontally-coadjacent windows. This prior color-selection structure has windows with a width-to-height ratio of about 0.46 and transmits about 44% or less of the electron beams.
When operating this latter device, the narrow vertical conductors are electrically positive with respect to the masking plate, so that the beamlets passing through each of the windows of the same aperture are deflected towards one another. Simultaneously, because of quadrupole-like focusing fields established in the windows, the beamlets are focused in the length direction of the phosphor stripes (compressed vertically) and defocused in the width direction of the phosphor stripes (stretched horizontally). The spacings and voltages are so chosen to form an electrostatic lens that also deflects the two beamlets to fall on the same phosphor stripe of the target.
The shapes of the deflected beamlets passing through each window, because they are elongated in the width (horizontal) direction and compressed in the length (vertical) direction, cause an overlapping of the beamlets onto the adjacent incorrect color phosphor stripe, or require a reduction in the widths of the windows, to assure adequate color purity in the image displayed on the target.
U.S. Pat. No. 4,316,126 issued to Hockings et al. on Feb. 16, 1982, overcomes the drawbacks of the former deflection-and-focus color-selection structure by disclosing a color-selection structure for a CRT which focuses the beamlets in the narrow width direction of the phosphor stripes and defocuses the beamlets in the long length direction of the phosphor stripes. In the latter color-selection structure, a metal masking plate has a array of apertures arranged in columns that are substantially parallel to the length of the phosphor stripes. An array of narrow conductors extend substantially parallel to the length of the stripes. The conductors are insulatingly spaced from the masking plate and are located opposite and spaced from the boundaries between adjacent triads. Each conductor is substantially centered over the apertures of one of the columns, so that the masking plate and the conductors define an array of windows for transmitting therethrough portions of the electron beams.
During the operation of the CRT using the latter color-selection structure, the polarities on the masking plate and the conductors are maintained so that the conductors are negative with respect to the masking plate. When so operated, the beamlets passing through each of the windows of the same apertures are directed away from one another. Beamlets from adjacent windows fall on the same stripe of the target. This requires the boundary of each triad, rather than the center of each triad, to be opposite the conductor. By using this arrangement of color-selection structure and screen and by operating the CRT in this manner, the transmitted beamlets are compressed (focused) in the direction normal to the lengths of the conductors and the phosphor stripes and are stretched (defocused) in the direction parallel to the lengths of the conductors and of the phosphor stripes. This reduces the widths of the beamlets and permits the transmission of the color-selection structure to be increased with improved registration of the beamlets on the phosphor stripes. The windows of the latter described color-selection structure have width-to-height ratios in the range of 0.8 to 1.1. With such ratios, transmission greater than 44% has been achieved without sacrificing color purity.
Neither of the aforedescribed color-selection structures provides means for producing a viewing screen or target having vertical phosphor stripes, using the color-selection structure as a master as is known in the art relating to shadow mask type CRT's and described in U.S. Pat. No. 4,049,451, issued to Law on Sept. 20, 1977.